Very high aspect ratio micro-electromechanical structures can be formed by the LIGA process. LIGA is a German acronym for the process steps of deep-etch x-ray lithography (LI), electroforming (G) and molding (A). In this process a layer of photoresist, typically about 0.05 to 2 mm thick, is deposited onto a metallic substrate or plating base, the photoresist is patterned by exposure to x-rays through a mask. Irradiated or unirradiated regions of the resist, depending on the type of photoresist used, are dissolved away by a developer solution to form a process mold. Electroplating using the conductive substrate as a cathode produces a complementary microstructure pattern. The remaining photoresist is dissolved leaving a patterned metal structure as the result.
LIGA process molds can be prepared using phenol-formaldehyde based photoresist materials and particularly the photoresist material designated SU-8. SU-8 is comprised of oligomers of phenol-formaldehyde (novolac) that have been quantitatively protected with glycidyl ether and w hose average epoxy functionality is 8. The photoresist is typically spun on substrates consisting of 75 nm Ti/400 nm Cu/75 nm Ti. However, for photoresist thicknesses useful for LIGA applications, typically greater than about 50-100 μm thick, the photoresist is prone to delaminate from the substrate either during development or subsequent electrodeposition stepd due to lack of adhesion between the photoresist and the substrate.
In an attempt to overcome the problem of lack of adhesion of epoxy-based photoresist materials to Ti/Cu/Ti substrates, and particularly SU-8 delamination, a wide variety of different coatings were examined by the inventor. Among the coatings tried were Cu, Si3N4, spin-on glass, hard baked novolac, and a polyimide bottom anti-reflecting coating. None were successful. Polymeric adhesion layers of crosslinked SU-8 and SU-8-like materials proved to be only sporadically successful. Successful repeated adhesion with these materials could not be correlated with any obvious experimental parameter. While the polymeric adhesion material bonded well with SU-8 it did not adhere to the Ti layer of the Ti/Cu/Ti substrate. The addition of Ti-based coupling agents to the polymeric adhesion layer composition or to the substrate itself also proved to be an unsatisfactory solution.